Two security researchers claim to have developed a new attack that can decrypt session cookies from HTTPS (Hypertext Transfer Protocol Secure) connections.

Websites use session cookies to remember authenticated users. If an attacker gains access to a user's session cookie while the user is still authenticated to a website, the hacker could use it to access the user's account on that website.

HTTPS should prevent this type of session hijacking because it encrypts session cookies while in transit or when stored in the browser. However, the new attack, devised by security researchers Juliano Rizzo and Thai Duong, is able to decrypt them.

Rizzo and Duong dubbed their attack CRIME and plan to present it later this month at the Ekoparty security conference in Buenos Aires, Argentina.

The attack exploits a weakness in a particular feature of the TLS (Transport Layer Security) cryptographic protocol and its predecessor, the SSL (Secure Sockets Layer) protocol, which are used to implement HTTPS.

All SSL and TLS versions are affected and the exploited feature is commonly used in SSL/TLS deployments, Rizzo said Thursday via email. The researcher declined to reveal which feature is vulnerable before the attack's presentation at Ekoparty.

The CRIME attack code, known as an agent, needs to be loaded inside the victim's browser. This can be done either by tricking the victim into visiting a rogue website or, if the attacker has control over the victim's network, by injecting the attack code into an existing HTTP connection.

CRIME doesn't require browser plug-ins to work; JavaScript was used to make it faster, but it could also be implemented without it, Rizzo said.

The attacker must also be able to sniff the victim's HTTPS traffic. This can be done on open wireless networks; on local area networks (LANs), by using techniques such as ARP spoofing; or by gaining control of the victim's home router through a vulnerability or default password.

For the attack to work, both the victim's client and the server hosting the targeted website need to support the vulnerable SSL/TLS feature, Rizzo said.

Rizzo confirmed that the HTTPS implementations on some popular websites are vulnerable to the attack, but declined to name any of them.

CRIME was tested successfully with Mozilla Firefox and Google Chrome. However, other browsers could also be affected, Rizzo said.

Mozilla and Google have already prepared patches that block the attack but they have not yet been released, the researcher said.

Last year at Ekoparty, Rizzo and Duong presented an attack called BEAST (Browser Exploit Against SSL/TLS), which was also capable of decrypting HTTPS session cookies. That attack affected SSL 3.0 and TLS 1.0 when used with certain cipher suites.

Mitigating BEAST involved upgrading to TLS 1.1 or 1.2, the latest versions of the TLS protocol, or prioritizing unaffected RC4-based cipher suites for older versions of the protocol.

However, the mitigation solution doesn't work for CRIME because, in addition to exploiting a feature that is present in all versions of SSL/TLS, the attack is not dependent on a particular cipher, Rizzo said.

According to data from SSL Pulse, a project that monitors SSL/TLS implementations across the Web, 72 percent of the Internet's top 184,000 HTTPS-enabled websites were still vulnerable to the BEAST attack at the beginning of August.